Thanks, she's a fruit of the night. The lm386 seems like a nice pick. I'm currently using a Texas TPA3123D2 which can produce as much as 25 watts per channel @ 4 Ohm with a 92% efficiency so there's practically no heat coming from it - it's pretty much a T-amp really.

As said, I still got a background signal and I'm now pretty sure its not a dc signal since it's chopped at about 300 Hz. I think it's the spi clock speed shifting between fast SD read and vs1053 writes. It make a short pause in between songs as I have som delays there.

So the clock speed switching probably generates spikes that are then somehow induced and carried by the outgoing audio signal. Any idea how can get rid of them, anyone?

So the clock speed switching probably generates spikes that are then somehow induced and carried by the outgoing audio signal. Any idea how can get rid of them, anyone?

Just the usual remedies...
- Avoid any vias for the RCAP reference capacitor.
- Keep the ground plane clean, always have ground plane below the analog signals, never cut the ground plane below an analog signal.
- Keep AVDD and IOVDD separate, have lots and lots of capacitors on both.
- If you must, then bring a separate ground trace along with the signal trace from the chip pin to the amplifier.

And finally remember that you must have the "sigma delta reconstruction" RC filter (470R+3.3nF) at the outputs if you drive any kind of amplifier or have a line out.

-Panu

PS. Hmmh... I am thinking of PCB layout matters more than anything. I'm not so sure with separate boards... just try to keep the power supplies separate and the ground clean... the ground is the most important signal since it's the only common reference...

Wow Panu, I'll just pop down a library and find a book to explain all the things you just told me

I'm not very skilled in electronics, but I do understand some of what you said. For now, I'll try modifications corresponding to the bits I did understand and skip the rest til PCB time. Thanks for all your help, meatime, here's a video of my project so far:

If the D-class amp can accept input signals of up to about 2.5V, then I'd connect GBUF to the IN- pin of the D-class amp and LEFT to the IN+ pin. That way you can have good sound quality with a differential connection to the differential amplifier and you don't need the DC blocking capacitors! If you look at download/file.php?id=45, you need the RC filters marked as "sigma delta reconstruction" and the "high frequency grounding path". But you can leave out the "DC block for output".

Hi Panu,
Is there any practical difference between connecting the analog outputs as suggested in this thread (LEFT -> INL+, RIGHT -> INR+, GBUF -> INL-, GBUF -> INR-) and inverting the signal as the USB Recorder schematic shows? And for the low pass filter, any benefit to the 1K/4n7 in the schematic versus 100/47n as suggested in TI's datasheet? Appreciate all the great info you've provided in these forums. Thanks!